The invention relates to noise-silencing mufflers.
The invention arose during continuing muffler development efforts, including further development efforts directed to the subject matter of the noted parent application.
The invention of the noted parent application arose during muffler development efforts, including those directed to solving problems in box-style mufflers, including muffler shell noise and poor muffler silencing. Since cost is almost always a concern, the solution to the two noted problems must also be cost effective. Box-style or stamped mufflers tend to radiate noise from their flat exterior surfaces. This characteristic is called shell noise and is most often a concern because of its harsh sound and adverse effects on muffler silencing. Also of concern with stamped mufflers is overall acoustic effectiveness. Because these types of mufflers are often constrained to a certain size and shape, their physical layout is not always conducive to good silencing.
The invention of the parent application addresses and solves the noted problems in a particularly cost effective manner using a simple design. In one aspect, the parent invention enables usage of identical parts within the muffler, which improves manufacturing efficiency and provides a cost reduction. Assembly of the muffler is also easy because the majority of the muffler's internal parts are designed into cross flow baffles. In accordance with the preferred embodiment, to combat the shell noise problem, the flow from the inlet is directed into one of two interior chambers of the muffler, formed by placing two of the cross flow baffles back to back. By letting the exhaust expand first in an interior chamber, the pressure pulses from the engine are less likely to cause exterior noise problems since they are damped considerably before reaching the muffler's outer shells. Stiffening bosses may be provided on larger flat areas of the baffles to control internal shell noise. To increase silencing capability, four chambers are created within the muffler by using a twin baffle design, along with two additional volumes between the outer shells and baffles. In one aspect, a horseshoe-shaped cross flow baffle is designed to provide the twin internal silencing chambers with desired flow path and area between them. The configuration increases the acoustical effectiveness of the muffler.
The present invention provides further improvements in both performance and lowered cost. The muffler design of the present invention provides optimization for the majority of small engine applications. In the preferred embodiment, as in the parent application, cost reduction is facilitated by the use of identical internal components. Performance gains are enabled by alternate flow routes designed into paired baffles, together with increased expansion chamber volume conducive to better silencing characteristics. The internal baffles divide respective chambers between themselves into a main chamber and subchambers and have respective sets of slots or apertures offset from each other and aligned with a respective subchamber. The offset forces the exhaust to turn as it travels into and out of the main chamber, enhancing acoustic silencing. Each baffle has a drawn center area dividing the volume between the outer shells of the muffler and the center chamber, allowing for more expansion and contraction of exhaust gas, enhancing acoustic silencing. An area between the top shell and the inner baffle provides a flow path forcing hot exhaust gas toward the surface of the top shell, enhancing cooling of the exhaust flow. The large surface area of the body helps minimize afterfiring, which is an undesirable bang or pop prevalent in small engines at shut down. Smaller drawn areas in the baffles provide additional chambers affording an alternate flow path for exhaust gas, lowering backpressure. The top shell is sloped for shedding debris, such as grass and dirt, which is desirable for lawn tractor applications.